Recycling of Elements Transported Upstream by Runs of Pacific Salmon: II. δ¹⁵N and δ¹³C Evidence in the Kvichak River Watershed, Bristol Bay, Southwestern Alaska

Abstract

Biota δ 15 N and δ 13 C values (deviations from recognized isotope standards) from lliamna Lake (a major anadromous sockeye salmon (Oncorhynchus nerka) nursery lake supporting peak-year runs > 10 million) and several other anadromous-salmon-free lakes in the Kvichak River watershed, Bristol Bay, southwestern Alaska, were compared to determine the significance of marine-derived nitrogen (MDN) delivered by returning adult salmon. Biota in lliamna Lake had higher δ 15 N compared with control lakes, verifying a mixing model correlating δ 15 N with MDN. Periphyton δ 15 N values reflected localized input from populations of spawning salmon. Juvenile sockeye MDN varied in response to escapement size, suggesting the importance of large escapements (> 10 million) for maintaining a predominantly MDN lacustrine N pool. Other resident fishes showed shifts in δ 15 N between years of high and low escapement. The dual-isotope approach, using δ 15 N and δ 13 C together, suggested that fish production is primarily dependent on limnetic primary and secondary production. The dual-isotope approach indicated that the coast range sculpin (Cottus aleuticus) was the only fish with an appreciable dietary component consisting of salmon eggs or emergent fry.